The invention relates to a modular system, to methods and apparatus for removably retaining the modules on a rigid underlying support surface, particularly to screening decks for screening particulate material and impact surfaces for diverting particulate material, the modules being removed and replaced easily as circumstances require. Another aspect of the invention includes an improved screening deck frame design.
Various screening decks and screening elements are known which are intended for the same general purposes. An example of a crowned screening element is shown in U.S. Pat. No. 4,120,784. The screening element is crowned in order to pretension the element for greater strength. However, crowned elements are difficult to remove and install and wear unevenly.
Various ways are known for connecting a screening element to an underlying frame to form a screening deck. Some methods require spacers or adapter bars between the screening element and the frame. An example of such an arrangement is shown in U.S. Pat. No. 4,409,099. That patent shows screening elements supported only at their corners on spacer elements so that the screening element is spaced above the frame, ostensibly to increase the effective screening area. The intermediate spacer elements add considerably to the cost and complexity, and increase the height of the overall screening apparatus. Additionally, specially formed frame members are required to reduce abrasion of frame members by the sifted particulate material. That requirement represents an expensive departure from standard practices in the industry.
Another screening apparatus that employs specialized components intermediate the screening elements and the support frame is shown in U.S. Pat. No. 4,219,412. There, a molded plastic frame is adapted to receive individual screening elements. The plastic frame, in turn, is connected to the frame. This combination leads to several disadvantages similar to the device of the U.S. '099 patent mentioned above. The '412 patent does not disclose whether or how individual screening modules might be removed and replaced.
Similarly, U.S. Pat. No. 4,661,245 shows a screening system that incorporates clamping bars which receive the individual screening elements. The clamping bars, in turn, are bolted onto the frame. The resulting structure is unduly complex, expensive, increases the height of the deck and suffers other shortcomings described below.
Several methods also are known for connecting flat screening elements directly to a supporting frame. One method is the use of a depending hook or skirt member integrally formed in the outlet side of the screening element. An example is shown in U.S. Pat. No. 3,980,555. According to that invention, depending hooks of abutting screening elements are inserted into a frame member through a common mounting hole. The hooks extend out of the mounting hole on the underside of the frame and engage the frame member to hold the screening elements in place. The resulting screening deck has a substantially contiguous inlet surface that provides no access to the mounting hooks for disconnecting an element which requires replacement. Accordingly, such arrangements require a user to climb under a screening deck and cut the depending hooks off of a module to be replaced. Or, more commonly, a user will pry the element off from the top (inlet side) and destroy it in the process. Either way, the module is rendered useless; a result that is acceptable for modules which are worn out, but wasteful if the module is being replaced merely to change sieve size.
Another attachment method is shown in U.S. Pat. No. 4,219,412, discussed above. There, individual screening elements are connected to an intermediate supporting frame by integrally formed depending hook-shaped members, similar to those disclosed in the U.S. '555 patent. Here, however, the depending hook members are inaccessible even from the outlet side of the screen, as they are enclosed within elongate U-shape channels. The '412 patent does not disclose any way of removing an individual screening element for replacement, short of disassembling the screening deck.
Another method of connecting screening elements to an underlying frame is to employ securing pins as suggested, for example, in U.S. Pat. No. 4,141,821. According to that invention, abutting screening elements each include hollow tubular protrusions extending through a common mounting aperture formed in the frame so as to define a hollow tube. A securing pin is inserted into the tube from the inlet side of the deck to spread the protrusions apart beneath the aperture and thereby engage the frame.
The securing pin taught in '821 is difficult to pry out of the tube. The pin perhaps could be driven out from the outlet side of the screen. In any event, once removed, the pin is loose and therefore subject to loss. This is a decided risk in the context of screening machines, as it often is impractical or impossible to retrieve an errant securing pin. A similar securing pin is employed in the apparatus disclosed in U.S. Pat. No. 4,409,099.
Yet another apparatus for removably mounting screening elements is disclosed in U.S. Pat. No. 4,670,136. That patent discloses a two-piece elongate clamping element for clamping the edge of a plastic module or wire mesh screen. The apparatus includes a first, lower plastic profile fastened to the frame and arranged to engage under the edges of at least two mutually adjacent screen elements. A second, upper profile is detachably fastened over the first lower profile for retaining the screen elements clamped in place. Removal of a screening element requires removal of the upper plastic profile, thereby subjecting it to risk of loss like the securing pins of the U.S. '821 patent and the like. Additionally, the clamping structures of the U.S. '136 patent extend substantially beyond the edges of the frame, thereby reducing the effective screening area and subjecting the clamping structure itself to excessive wear from abrasion by particulate material being screened.
Another problem common to known screening apparatus is destructive abrasion of frame members by particulate material that passes through screening elements. Such material abrades exposed portions of the frame members after it exits the outlet side of the module adjacent frame members. U.S. Pat. No. 4,409,099 above teaches use of a special coating and a specially formed sectional shape of the frame member to ameliorate this problem. However, coating adds significantly to the manufacturing cost of the frame with questionable improvement in frame life. The specialized frame member shape requires custom material and undesirably increases the height of the screening deck.
Accordingly, the need remains for a system for screening particulate material which provides for fast, convenient and nondestructive replacement of screening modules and avoids abrasive wear of the underlying support surface. There also remains a need for a system for diverting particulate material which includes impact elements which are fast and convenient to remove and replace with respect to the support structure.